High-strength bolts may be used in either slip-critical or bearing-type connections (Art. 5.3), subject to various limitations. Bearing-type connections have higher allowable loads and should be used where permitted. Also, bearing-type connections may be either fully tensioned or snug-tight, subject to various limitations. Snug-tight bolts are much more economical to install and should be used where permitted.
Bolted slip-critical connections must be used for bridges where stress reversal may occur or slippage is undesirable. In bridges, connections subject to computed tension or combined shear and computed tension must be slip-critical. Bridge construction requires that bearing type connections with high-strength bolts be limited to members in compression and secondary members.
Carbon-steel bolts should not be used in connections subject to fatigue. In building construction, snug-tight bearing-type connections can be used for most cases, including connections subject to stress reversal due to wind or low seismic loading. The American Institute of Steel Construction (AISC) requires that fully tensioned high-strength bolts or welds be used for connections indicated in Sec. 6.14.2.
The AISC imposes special requirements on use of welded splices and similar connections in heavy sections. This includes ASTM A6 group 4 and 5 shapes and splices in built-up members with plates over 2 in thick subject to tensile stresses due to tension or flexure. Charpy V-notch tests are required, as well as special fabrication and inspection procedures. Where feasible, bolted connections are preferred to welded connections for such sections (see Art. 1.17).
In highway bridges, fasteners or welds may be used in field connections wherever they would be permitted in shop connections. In railroad bridges, the American Railway Engi- neering Association (AREA) recommended practice requires that field connections be made with high-strength bolts. Welding may be used only for minor connections that are not stressed by live loads and for joining deck plates or other components that are not part of the load-carrying structure.
BOLTS IN COMBINATION WITH WELDS
In new work, ASTM A307 bolts or high-strength bolts used in bearing-type connections should not be considered as sharing the stress in combination with welds. Welds, if used, should be provided to carry the entire stress in the connection. High-strength bolts propor- tioned for slip-critical connections may be considered as sharing the stress with welds.
In welded alterations to structures, existing rivets and high-strength bolts tightened to the requirements for slip-critical connections are permitted for carrying stresses resulting from loads present at the time of alteration. The welding needs to be adequate to carry only the additional stress.
If two or more of the general types of welds (groove. fillet, plug, slot) are combined in a single joint, the effective capacity of each should be separately computed with reference to the axis of the group in order to determine the allowable capacity of the combination.
AREMA does not permit the use of plug or slot welds but will accept fillet welds in holes and slots.
FASTENERS
In steel erection, fasteners commonly used include bolts, welded studs, and pins. Properties of these are discussed in the following articles.